Preparation of printing plates employing organic polymerizable photoconductor

ABSTRACT

A process for the preparation of printing plates from electrophotographic reproduction material, having a support suitable for printing purposes and a photoconductive layer thereon, by charging, imagewise exposure to light, developing the latent electrostatic image on the layer with a finely divided toner, subsequent heating, and removal of the photoconductive layer in the nonimage areas. The latent electrostatic image is produced on a layer which contains at least one known polymerizable organic photoconductor containing or forming during heating olefinic double bonds, and the image is made visible by means of a developer which, at least partially, consists of one or more finely divided solid substances forming radicals when heated. The reproduction material with the developed image thereon is heated to a temperature between 50* and 300* C., preferably to a temperature between 100* and 200* C., and finally the organic photoconductive layer is removed, in known manner, in the nonimage areas by dissolving it away.

United States Patent [72] Inventor lErwin Lind Auringen ueber Wiesbaden,Germany [21] Appl. No. 637,639 [22] Filed May 11, 1967 [45] PatentedOct. 26, 1971 [73] Assignee Kalle Aktiengesellschaft Wiesbaden-Biebrich,Germany [32] Priority May 13, 1966 [33] Germany [31] K 59245 [54]PREPARATION OF PRINTING PLATES EMPLOYING ORGANIC POLYMERIZABLEPHOTOCONDUCTOR 6 Claims, No Drawings [52] U.S.Cl 96/1, 101/456,101/471,260/886,260/844 [51] Int. Cl. G03g 5/00, B41n 1/00, B41n 5/00 [50] FieldofSearch 101/1492; 96/1,1.5,1.2

[56] References Cited UNITED STATES PATENTS 3,418,116 12/1968 lnamietal.96/1.5 3,231,374 1/1966 Sciambi 96/1 Primary Examiner-George F. LesmesAssistant Examiner-M. B. Wittenberg Attorney-James E. Bryan ABSTRACT: Aprocess for the preparation of printing plates from electrophotographicreproduction material, having a support suitable for printing purposesand a photoconductive layer thereon, by charging, imagewise exposure tolight, developing the latent electrostatic image on the layer with afinely divided toner, subsequent heating, and removal of thephotoconductive layer in the nonimage areas. The latent electrostaticimage is produced on a layer which contains at least one knownpolymerizable organic photoconductor containing or forming duringheating olefinic double bonds, and the image is made visible by means ofa developer which, at least partially, consists of one or more finelydivided solid sub stances forming radicals when heated. The reproductionmaterial with the developed image thereon is heated to a temperaturebetween 50and 300C, preferably to a temperature between 100 and 200 C.,and finally the organic photoconductive layer is removed, in knownmanner, in the nonimage areas by dissolving it away.

IPREPARATHON F rmnrnso PLATES EMPLUYIN G ORGANIC POLYMEMZABLEFHOTUHEIQNDUCTOR It is known to use photoconductive organic compounds inthe photoconductive, layers of of electrophotographic reproductionmaterials. Furthermore, it is known to process electrophotographicreproduction materials containing organic photoconductors into printingplates. These latter have supports suitable for printing purposes. Inthe unprocessed state, they also are called electrophotographicmaterials. An electrophotographic material is known, for example, thephotoconductive layer of which consists of a monomolecularphotoconductor and high-molecular weight alkali-soluble sub stances.This material is charged, exposed to light, and developed with dyedresin powder in the usual manner. The resin powder is fixed by burningin, the layer is then treated with an alkaline solution to remove thenonimage areas, and a planographic printing plate results. Thedisadvantage of this process is that, when treating with the alkalinesolution, comparatively small image areas are easily undermined andsometimes even washed away.

Furthermore, an electrophotographic material is known thephotoconductive layer of which contains zinc oxide finely distributed ina cross-linkable binding agent. A method is known for the processingthereof where for developing the latent electrostatic image, a substancecatalyzing cross-linking of the binding agent is used in a finelydistributed solid form. When burning in the developer, the image areasbecome difficulty soluble or insoluble in certain solvents in which thenonimage areas, which are not cross-linked, are readily soluble. For thepreparation of a printing plate, the photoconductive layer is dissolvedaway in the nonimage areas. Here, too, the already mentioned effect ofundermining the image areas occurs since cross-linking of the layer doesnot uniformly penetrate to the support because of the incorporated zincoxide. in order to overcome this drawback, in a variation of theprocess, electrophotographic printing materials are used which, betweenthe support and the photoconductive layer, additionally contain a metalchromate or phosphate intermediate layer which increases adhesion.Undermining of the image areas is considerably reduced by this measurebut the process still has the disadvantage that organic solvents must beused for removing the nonimage areas.

The present invention provides a process for the electrophotographicpreparation of printing plates which meet high requirements regardingdissolving ability and length of printing run and the nonimage areas ofwhich preferably can be removed by means of an aqueous solution.

According to the present process, a planographic printing plate isobtained which may be converted, by means of one of the known processes,into a relief printing plate or, if desired, also into an intaglioprinting plate, if the support material is appropriate.

The essence of the invention is that a polymerizable monomericphotoconductor is imagewise polymerized by radical initiation, theimagewise distribution of the radical donor being achieved by its use asa developer substance.

The electrophotographic material serving as starting material in theprocess is known in principle. The use of a photoconductive organicsubstance having an olefinic double bond is known. The use thereof inprinting plates is not expressly described, but it is known from GermanPat. No. 1,117,391, that they are, in principle, useful in this field.

In the process according to the invention, there may be usedphotoconductors having vinyl or vinylidene groups as well as those inwhich three of all four hydrogens of the ethylene group are substituted.Suitable polymerizable compounds also include photoconductive stilbeneand styrene derivatives.

Compounds carrying vinyl groups preferably are employed. Vinylcarbazole,substituted vinylcarbazole, substituted vinyloxazoles, substitutedvinyltriazoles, and vinyldibensofuran have proved particularly suitable.Suitable compounds which do not form the vinyl group before heating arethe substituted hydroxyethyl-diphenyltriazoles. They split off waterduring heating, whereby a vinyl group is formed.

A very suitable compound including the vinylidene group isacenaphthylene.

insofar as the polymerizable organic photoconductors are sufficientlyfilm forming themselves, they may be directly ap plied to a suitablesupport and processed according to the process of the invention. Sincethis usually is not the case to a sufficient extent, it has provedadvantageous to employ a material the layer of which, in accordance withGerman Pat. No. 1,117,391, in addition to the organic photoconductor,contains resins which are soluble in aqueous alkali. Suitable resins arehigh molecular weight substances carrying alkalisolubilizing groups.Such groups, for example, are acid anhydride, carboxylic, sulfonic,sulfonamide, or sulfonamide groups. Preferably employed are resinshaving high acid numbers since they are especially soluble in aqueousalkaline solutions. Copolymers having anhydride groups can be usedparticularly successfully since, by the absence of free acid groups, theconductivity in the dark of electrophotographic layers is low despitethe good alkali-solubility thereof. Copolymers of styrene and maleicanhydride have proved particularly suitable.

A preferred embodiment of the process according to the in ventiontherefore is to produce the latent electrophotographic image on a layercontaining at least one polymerizable organic photoconductive compoundhaving a vinyl group, and an alkali-soluble binding agent. Afterdevelopment and heating, the imagefree areas of the photoconductivelayer are removed in known manner by treatment with an aqueous alkalineliquid, if desired containing an organic solvent. Suitable alkalineliquids are described in the German Pat. No. 1,] 17,391.

The layer of the electrophotographic starting material also may containsensitizers, in known manner. These are added to the reproduction layerin small quantities, about 0.0000l to about 0.1 percent, calculated onthe weight of photoconductor. Suitable sensitizers, generally dyestuffs,are known from Belgian Pat. No. 558,078, for example.

Furthermore, the light-sensitivity of the layer may be increased, inknown manner, by the addition of so-called activators. These areelectron acceptors which form molecular complexes with thephotoconductors, which generally are electron donors. Particulars aredisclosed in German Pat. No. 1,127,218.

As layer supports for use in the electrophotographic starting material,all materials known for this purpose may be used, e.g. aluminum, zinc,magnesium, or copper plates, and also cellulose products, such asspecial papers, cellulose hydrate, cellulose acetate, or cellulosebutyrate films, the latter particularly in a partially saponified form.To a certain extent, plastic materials, such as polyamides in the formof films, also are suitable as supports.

For the performance of the process of the invention, anelectrophotographic material of the above-described type iscorona-charged in the usual manner. Positive or negative chargesoptionally may be sprayed thereon. A latent electrostatic charge imagethen is produced on the charged layer by means of imagewise exposure tolight. Contact exposure, or diascopic or episcopic exposure is possible.it is also possible to employ a contact or glassengraved screen. Thelatent image then is made visible by means of a developer which at leastcontains substances which decompose into radicals when heated.

Suitable developers are all available solid compounds which find use inradical-initiated polymerization of olefins. in the present case, thefollowing have proved particularly advantageous: benzoyl peroxide,dichlorobenzoyl peroxide, succinyl peroxide, azoisobutyric nitrile,tetraphenyl hydrazine, dehydrolophine, diazidostilbene, and sodiumtrichloroacetate. These substances may be used for development either inthe dry state or dispersed in the form of a liquid developer.

The invention, therefore, also provides a dry finely divided developerpowder for the performance of the process of the invention, whichconsists entirely or partially of one or more substances which formradicals when heated.

Generally, only the pure radical-forming substance is employed as adeveloper. If, however, the image is to be made distinctly visible, anormal resin-containing toner may be mixed therewith. The proportion ofradical-forming substance in the dry developer should, however, not bebelow 20 percent by weight. Furthermore, it has proved advantageous topulverize the radical-forming substances to a particle size less than/1..

In most cases, the developer substance is applied to the layer to bedeveloped by means of a support. A mixture with iron filings used in theknown magnetic brush or magnetic roll leads to favorable results.

Due to clean working and high-dissolving power, liquid development hasproved particularly advantageous in the present invention. Theinvention, therefore, also provides a liquid developer containing ahighly resistant liquid phase and a finely divided solid phase dispersedtherein, the solid phase entirely or partially consisting of one or moresubstances forming radicals when heated. Here, too, the preferablyemployed solid substance is a pure radical-forming substance Sometimes,however, it is of advantage when the solid phase includes a dyestuff.The particle size also should be below a. The solid substance usually isground in a ball mill.

As dispersing agents, there are employed liquids which do not dissolvethe solid phase. Suitable are, for example, some hydrocarbon halides,particularly, however, higher molecular weight, liquid aliphatichydrocarbons, such as the product known under the name Shellsol T(registered trademark). The charge nature of the dispersed solid phasedepends upon the properties of the radical-forming substance itself aswell as upon the specific solvent used. When dispersed in Shellsol, theradical-forming substances exhibit the following nature of charge:diazidostilbene and sodium trichloroacetate are given negative charges;all the other substances, however, are given positive charges.

It is possible, however, to change or stabilize the nature of the chargein a known manner by means of additives. Suitable additives arenaphthenates, for example, and they are particularly advantageouslyemployed together with a small quantity of binding agent, such aslinseed oil. Their technical effect is known from German Pat. (DAS) No.1,047,516.

Application of the liquid developer may be performed in a known manner,e.g. by immersion or roller application. Preference is given, however,to roller application because of higher uniformity achieved therewith.

After development of the latent electrostatic image with one of theaforementioned developers of the invention, the radical-formingsubstance loosely adheres imagewise to the polymerizable layer.Polymerization then is initiated by heating. Polymerization is effected,as already stated, in the temperature range between 100 and 200 C. Thespecific temperature advantageously depends upon the decompositiontemperature of the radical-forming substance employed. Heatingpreferably performed in an oven heated to the required temperature,Infrared radiators of sufficient intensity also are suitable; whenemploying them, it is advisable, however, to use a dyed developer inorder to increase the energy reception of the developed image areas. Theduration of the heat treatment depends upon the photoconductor to bepolymerized, the radical-forming substance which initiatespolymerization, the temperature employed, and the thickness of the layerto be polymerized. Sometimes, heating for 30 seconds is sufficient topolymerize the layer; sometimes, however, several minutes are required.The extreme duration of heating is 5 minutes.

The image areas polymerized by heating become insoluble in suitablesolvents, i.e. in the preferred embodiment they are insoluble in aqueousalkaline solutions.

The solutions are applied to the layer, e.g. by means of a cotton pad.It also is possible to immerse the plates directly into the liquid. Alsosuitable for removing the nonimage areas are appropriately constructeddevices, e.g. those with application rollers for the liquid.Differentiation into hydrophilic and oleophilic areas, which is desiredin offset printing, is thereby obtained, the polymerized organicphotoconductive layer yielding the oleophilic areas and the supportsurface yielding the hydrophilic areas.

After treatment with the alkaline liquid, the printing plateadvantageously is afterrinsed with water and, if desired, thehydrophilic properties are further enhanced by wiping over with dilutephosphoric acid solution. After inking up with greasy ink, prints can bemade therefrom in planographic printing machines in known manner.

It also is possible to produce printing plates for relief and, ifdesired, also for intaglio printing by subsequent etching of the baredsupport. The etching is performed in known one-step or multistep etchingmachines.

The printing plates prepared in accordance with the invention using oneof the described developers produce long runs. The photoconductivelayer, which generally has a thickness of I only a few microns iscompletely polymerized to the depth of the support and is firmlyanchored therewith. Undermining during removal of the nonimage areasdoes not occur. Since the organic photoconductive layers arehomogeneous, grainless layers, the finest screen dots easily can betruly reproduced. The process of the invention, therefore, alsopreferably is employed in the preparation of printing plates of veryfinely screened halftone originals. This field hitherto has beensubstantially foreclosed to electrophotographic printing plates.

The invention will be further illustrated by the following examples:EXAMPLE 1 A superficially roughened about 4. thick aluminum foil iscoated with a solution of2 g. of N-dimethylvinylcarbazole, 2 g. ofapolystyrene synthetic resin (Lytron 820 of Monsanto), and 4 mg. ofRhodamine B in 40 g. of glycol monomethylether, and dried. The coatedmaterial is positively charged to about 300 volts by means of a coronadischarge. Then, exposure is performed under a positive screened filmoriginal in a vacuum frame. When using a 200 watt incandescent lamp at adistance of 75 cm., the exposure time is 2 seconds. The charge imageproduced on the photosemiconductor layer then is developed in adeveloper bath which contains 10 g. of finely distributed sodiumtrichloroacetate per 100 g. of a high-boiling hydrocarbon (Shellsol T).After evaporation of excess dispersing agent adhering to the layer. theresulting white image of sodium trichloroacetate is heated for 3 minutesto 200 C., the sodium trichloroacetate being thereby decomposed. Theradicals resulting during decomposition polymerize thephotosemiconductor layer of N- dimethylvinylcarbazole in the imageareas. The layer is removed in the nonimage areas by bathing it for 2minutes in a solution of 5 g. of sodium metasilicate in 100 ml. ofwater, 40 ml. of methanol, 35 ml. of glycol, and 20 ml. of glycerol, andis subsequently is wiped with a sponge. A printing plate is thusobtained by means of which very long run of several ten thousands ofprints can be achieved in the offset printing process. EXAMPLE 2 Insteadof the solution of N-dimethylvinylcarbazole used in example 1, asolution ofa corresponding quantity of 2-vinyl-4-(o-chlorophenyl)-5-(p-diethylaminophenyl)-oxazole is used for coating analuminum foil. The layer is negatively charged to 400 volts by means ofacorona device. Exposure is carried out under a contact screen and adiapositive, the exposure time depending upon the specific source oflight used and the enlargement scale. Usually, it is in the range ofseveral seconds. The exposed plate is developed with a liquid developerwhich contains 6 g. of finely distributed pure tetraphenyl hydrazine in1,000 g. of a high-boiling hydrocarbon (Shellsol T). The remainder ofthe process is as in example l. A screened printing plate is obtained.EXAMPLE 3 An etchable zinc plate is coated with a solution of 10.5 g. of2-vinyl-4-(o-chlorophenyl)-5-(o-diethylaminophenyl)-oxazole, 25 g. ofanovolak (Alnovol 429 K), 20 g. ofa polyvinyl acetate (Mowilith CT and120 mg. of Rhodamine B in 300 g. of glycol monoethyl ether, and dried.The thickness of the dry photoconductive layer should be about to 12 1..It is negatively charged to about 400 volts by means of a corona device.Exposure may be performed in a contact reproduction frame, as describedin example 1'. The latent charge image produced by exposure is developedwith a dispersion of 10 g. of dehydrolophine in 1,000 g. of ahigh-boiling hydrocarbon (Shellsol T), and is then heated to 200 C. anddecoated. The zinc bared by decoating in the nonimage areas then may bedeep-etched by means of commercial etching media. A relief printingplate is thus obtained.

EXAMPLE 4 A superficially roughened aluminum foil is coated with asolution of 5 g. of l-hydroxyethyl-2,5-bis-(l-diethylaminophenyl)-l,3,4-triazole, 5 g. of polystyrene syntheticresin (Lytron 820 of Monsanto), and 10 mg. of Rhodamine B in 100 g. ofglycol monoethyl ether. The layer is negatively charged to 400 volts.Exposure is performed with a reproduction camera having eight 500 wattincandescent lamps. The original is a halftone photograph. Exposure iscarried out under a glass-engraved screen and the plate, which has beenexposed to light for 60 seconds, is developed in the manner describedabove with a dispersion of 20 g. of benzoyl peroxide in 1,000 g. of ahigh-boiling hydrocarbon (Shellsol T). Heating is performed for about 5minutes, a somewhat longer period than is usual with vinyl compoundssince water first has to be split from the hydroxyethyl group. Afterdecorating the nonimage areas, a printing plate for offset printing isobtained.

It will be obvious to those skilled in the art that many modificationsmay be made within the scope of the present in vention without departingfrom the spirit thereof, and the invention includes all suchmodifications.

What is claimed is:

i. A process for the preparation of a printing plate which comprisesexposing an electrostatically charged, supported photoconductiveinsulating layer to light under a master, the layer comprising amonomeric polymerizable organic photoconductor containing an olefinicbond, developing the resulting latent electrostatic image by contactingit with a finely divided material which forms free radicals when heated,heating the layer to a temperature in the range of about 50300 C. topolymerize the photoconductor, and removing the photoconductive layerfrom the support in the nonimage areas.

2. A process according to claim 11 in which the photoconductiveinsulating layer comprises a polymerizable organic photoconductorcontaining vinyl groups in admixture with an alkali-soluble bindingagent, and the photoconductive layer is removed from the support in thenonimage areas by treatment with an aqueous alkaline liquid.

3. A process according to claim 1 in which the finely divided materialwhich forms radicals when heated is contacted with the latentelectrostatic image in the form of a dry powder.

4. A process according to claim 1 in which the finely divided materialwhich forms radicals when heated in contacted with the latentelectrostatic image in the form of a dispersion in a dielectric liquid.

5. A process according to claim l in which the polymerizable organicphotoconductor is selected from the group consisting of vinylcarbazoles,vinyloxazoles, vinyltriazoles, vinyldibenzofuran, andhydroxyethyl-diplhenyltriazoles.

6. A process according to claim 1 in which the finely divided materialwhich forms radicals when heated is selected from the group consistingof benzoyl peroxide, dichlorobenzoyl peroxide, succinyl peroxide,azoisobutyric nitrile, tetrapltenyl hydrazine, dehydrolophine,diazidostilbene, and sodium trichloroacetate.

2. A process according to claim 1 in which the photoconductive insulating layer comprises a polymerizable organic photoconductor containing vinyl groups in admixture with an alkali-soluble binding agent, and the photoconductive layer is removed from the support in the nonimage areas by treatment with an aqueous alkaline liquid.
 3. A process according to claim 1 in which the finely divided material which forms radicals when heated is contacted with the latent electrostatic image in the form of a dry powder.
 4. A process according to claim 1 in which the finely divided material which forms radicals when heated in contacted with the latent electrostatic image in the form of a dispersion in a dielectric liquid.
 5. A process according to claim 1 in which the polymerizable organic photoconductor is selected from the group consisting of vinylcarbazoles, vinyloxazoles, vinyltriazoles, vinyldibenzofuran, and hydroxyethyl-diphenyltriazoles.
 6. A process according to claim 1 in which the finely divided material which forms radicals when heated is selected from the group consisting of benzoyl peroxide, dichlorobenzoyl peroxide, succinyl peroxide, azoisobutyric nitrile, tetraphenyl hydrazine, dehydrolophine, diazidostilbene, and sodium trichloroacetate. 